1. Field of the Invention
The present invention relates to electric power conversion devices comprised of a transformer, a direct current-alternating current conversion circuit, an alternating current-direct current conversion circuit and a control circuit.
2. Description of the Related Art
There are known direct current-direct current converters (DC-DC converters). The DC-DC converter is comprised of a direct current-alternating current conversion circuit (DC-AC conversion circuit) and an alternating current-direct current conversion circuit (AC-DC conversion circuit). For example, patent document, Japanese patent laid open publication No. JP 2009-118571, which has disclosed a DC-DC converter.
The DC-DC converter further has a control circuit equipped with a central processing unit (CPU), a comparator, a drive circuit, etc. The CPU generates a target current instruction value on the basis of a difference between an output voltage of the DC-DC converter and a target voltage value. Further, the CPU generates and outputs a predetermined duty ratio and a plurality of pulse signals having a predetermined frequency. The comparator compares an absolute value of a current flowing in the transformer with the target current instruction value. When the comparison result of the comparator indicates that the absolute value of the current flowing in the transformer is smaller than the target current instruction value, a drive circuit in the control circuit outputs the pulse signal transmitted from the CPU as the drive signal to drive switching transistors, for example IGBTs.
On the other hand, when the comparison result of the comparator indicates that the absolute value of the current flowing in the transformer is larger than the target instruction current value, the comparator inhibits the pulse signals transmitted from the CPU to be used for driving the switching transistor. The drive circuit controls the operation of the switching transistors on the basis of the drive signals, i.e. the pulse signals output from the CPU. This control operation makes it possible to correctly adjust the output voltage of the DC-DC converter to the target voltage.
By the way, when the DC-DC converter enters an abnormality state, i.e. a malfunction, the DC-DC converter cannot correctly adjust the output voltage of the DC-DC converter to the target voltage. That is, looking from different angles, it is possible to detect the abnormality state of the DC-DC converter on the basis of a condition whether or not the output voltage of the DC-DC converter correctly equals to the target voltage. However, even if the DC-DC converter correctly works, the output voltage of the DC-DC converter is decreased when the output current of the DC-DC converter exceeds its rated current. In this case, it is difficult to correctly detect that the abnormality state of the DC-DC converter enters the abnormality state or a malfunction on the basis of the judgment result whether or not the output voltage exceeds its target voltage.
In order to solve the conventional problem previously described, there has been proposed a conventional technique which detects whether or not an output current of the DC-DC converter exceeds its rated current. The conventional technique judges occurrence of the abnormality state of the DC-DC converter on the basis of the detection result. However, this conventional technique requires one or more additional circuits for detecting the output current of the DC-DC converter. As a result, the DC-DC converter has a complicate structure and a manufacturing cost thereof increases.
There has been proposed another conventional technique which detects an output current of the DC-DC converter on the basis of a current flowing in the transformer, and judges whether or not the detected output current exceeds its rated current in order to judge occurrence of the abnormality state of the DC-DC converter. However, it is also difficult to correctly and precisely detect the output current of the DC-DC converter because characteristics of each of components forming the DC-DC converter vary due to a temperature and its production tolerance. Furthermore, it is necessary to perform a complicated process and needs a high performance CPU in order to detect the output current of the DC-DC converter with high accuracy.